(1) Field of Invention
This invention is directed to highly selective anion exchange membranes prepared from aqueous solutions of quaternary ammonium methacrylic esters in a polar solvent. The membranes which are especially useful in electrodialysis applications are obtained with active exchange sites in a "one-step" procedure and require no further processing.
(2) Description of the Prior Art
The usual technique for the production of anion selective membranes similar to those described in the invention involves co-polymerizing methacrylate esters containing amine groups of the tertiary type, with cross-linking methacrylate esters. The resulting polymer with pendant tertiary amine groups is further reacted so that the tertiary amine groups are converted to the quaternary ammonium salts producing an anion selective membrane structure. The prior art has disadvantages in that the polymerization reaction is carried out in non-water soluble organic solvents that have to be washed out with a water soluble alcohol, such as methanol, before the polymer sheet can be subjected to an alkylation step which involves quaternization to the ammonium salt.
In U.S. Pat. No. 4,231,855 (Hodgdon, et.al.), which is incorporated herein by reference, there is disclosed such a process for synthesizing aliphatic quaternary ammonium ion-exchange polymers. The method involves the polymerization of a non-ionic methacrylate ester monomer containing tertiary amine groups such as, for example, dimethylaminoethyl methacrylate (DMAEMA) with a non-ionic polyunsaturated cross-linking methacrylate ester monomer containing vinyl groups such as ethylene glycol dimethacrylate (EGDM) in an appropriate non-water soluble organic solvent. The resulting non-ionic polymer is then made ionic by reacting the polymer with an alkylating reagent such as methyl chloride (CH.sub.3 Cl) to convert the tertiary amine groups to the quaternary ammonium groups.
The present invention comprises an improvement over the methods of the prior art by using water soluble polar solvents. The methanol washings of the prior art (to remove the organic solvent) as well as the second procedural step whereby pendant tertiary amine groups are transformed into guaternary ammonium salts using hazardous alkylating reagents (such as methyl chloride gas) are eliminated. The resulting anion exchange membranes have high ion-exchange capacities and low electrical resistance. Additionally, the use in the present invention of polar non-polymerizable solvents which can be removed from the polymer by water washes instead of methanol washes, and elimination of the alkylation step represents a large cost savings. Also, the elimination of methyl chloride gas is an added cost savings, as well as a solution to an environmental problem that under present regulation requires strict handling of hazardous waste.